Touch sensitive input surfaces such as touch sensitive computer display screens and digitizing tablets are finding increasing use in applications which make use of "user-friendly" computer interfaces. Computer systems which include a touch sensitive computer display screen, hereinafter "touchscreen", employ a controller which provides an interface between the computer and the touchscreen. A library of controller interface routines stored in the computer provides an application program with a set of commands which may be used by the application program to initialize the controller, select operating modes, specify data formats and execute diagnostic functions.
Capacitive type touchscreens which generate a uniform electrical potential across the surface of the screen are seeing increasing use due to the numerous advantages offered by such screens. Capacitive type touchscreens may be used with a variety of types of touch devices, such as touch pens and human fingers so long as the touch device is sufficiently conductive.
In instances where the touchscreen accepts touch devices such as a touch pen and a finger, it is often desirable to distinguish between inputs entered via the pen and inputs entered via contact of the finger. For instance, when a user is writing on the screen with a pen, portions of the user's hand or wrist may be touching the screen. In such a situation, it is particularly important to distinguish between the inputs entered by contact of the pen and inputs entered by inadvertent contact of the hand. Applications in which pen and finger inputs are accepted to perform different functions also benefit from a system which distinguishes between pen and finger inputs.